The low-pressure area (LPA) east of the country may develop into a tropical depression by Wednesday or Thursday.
The Philippine Atmospheric, Geophysical, and Astronomical Services Administration estimated the location of the LPA at 320 kilometers east of Catarman, Northern Samar around 10 a.m. Monday.
It is forecast to traverse Visayas and Southern Luzon as an LPA and develop into a tropical depression once it moves over the West Philippine Sea, the State weather bureau said.
As the LPA is seen to traverse the archipelago, it is expected to bring cloudy skies with light to moderate with at times heavy rains over Albay, Camarines Norte, Camarines Sur, Catanduanes, Sorsogon, Masbate, Biliran, Eastern Samar, Leyte, Northern Samar, Samar, and Southern Leyte as early as Monday.
Scattered rains may also affect Marinduque, Romblon, Quezon, Mindanao, and the rest of Visayas.
"Flooding and rain-induced landslide may occur during heavy or prolonged rainfall especially in areas that are highly or very highly susceptible to these hazards," PAGASA warned.
Metro Manila and the rest of Luzon will still be warm and humid in the morning with possible isolated rain showers in the afternoon or evening due to localized thunderstorms within the day.
PAGASA advised the public to remain vigilant against possible flash floods or landslides during severe thunderstorm activity.
The State weather bureau has not issued a gale warning Monday thus fisherfolk and those using small boats are safe to sail. Slight to moderate sea condition will prevail across the country's coastlines.
If the LPA intensifies into a tropical cyclone while inside the Philippine Area of Responsibility, PAGASA will be giving it a local name "Leon."
Based on the PAGASA's historical record, two to three tropical cyclones usually enter or form inside the PAR in September. So far, one tropical cyclone named "Kristine" has entered the PAR this month.
Weather specialist Aldczar Aurelio said the weather disturbance may bring rain showers and thunderstorms over Metro Manila in the next three days.
PAGASA's extended weather outlook for selected areas also shows that rain showers and thunderstorms may prevail over Tagaytay City, Lipa City, Legazpi City, Puerto Princesa City, Iloilo City, Bacolod City, Cebu City, Tacloban City, Cagayan de Oro City, Valencia City, Davao City, and Zamboanga City in the next few days.